Option 2: Configure IIS/WebDAV and expose the box hosting calendar to the world/internet (Security concern and not sure if it'd work with "Annonymous Authentication" ...)
slipstick.com/outlook/ol2007/publishcal.htm]Share Calendars with Local Users using Internet Publishing

I have been researching replacing our current Exchange with open source tools for when we inevitably will need to upgrade. Citadel (I'm too new to post their website, google "Citadel email" and it should be your first hit) seems to be an easy to install and very complete mail package complete with active directory integration and webmail capabilities with shared calendars. I did a basic configuration reading off my current active directory systems in about 30 min, including the install of Ubuntu 8.04 server. I can't speak to security since I have not done extensive testing yet.

I have also heard great things about Zimbra as a mail front, but I have not used it yet.